Article written by: Jason Danaher Ph.D.
Flow switches and flow meters are commonly used in aquaculture systems to monitor and manage water movement. Although both devices can detect flow, they provide different types of information and serve different purposes within a system. A flow switch provides a simple signal when a predetermined flow condition is met, while a flow meter measures and reports the actual flow rate. Understanding the difference between these devices can help ensure the appropriate equipment is selected for system monitoring, control, and protection.
Flow Switch
For system design, a flow switch is appropriate when the primary requirement is to confirm that a minimum flow condition has been met. A flow switch is an ON/OFF device. It detects when fluid is moving below a predetermined flow threshold. When the flow reaches the set point, the switch changes state and sends a signal to another device or control system. If flow stops or falls below the threshold, it can trigger an alarm, shut down equipment, or prevent equipment from operating.
Figure 1. An in-line flow switch used to detect minimum water flow requirements for safe operation of equipment.
Flow switches are therefore primarily used as safety devices during equipment operation. They can help protect equipment from conditions such as pump failure, closed valves, loss of circulation, or insufficient flow. However, a flow switch does not provide the actual flow rate once its threshold has been reached.
A typical flow switch application in aquaculture is to install the device on the inlet side of an ultraviolet light (UV) unit. The flow switch detects whether the minimum required flow has been established and sends a signal to the UV unit or ballast, allowing the UV lamp to operate while water is flowing through the UV housing.
If the primary pump is turned off or the UV unit is bypassed and the operator forgets to turn off the lamp, the flow switch detects the loss of flow and signals the system to shut off power to the UV lamp. This prevents excessive heat from the lamp from accumulating inside the UV housing, which could damage the unit and, in extreme cases, create a fire hazard.
Flow Meter
For system design, a flow meter is more appropriate when knowing the actual flow rate is important for system operation, monitoring, control, or documentation. A flow meter provides a continuous measurement of the actual flow rate, such as 25 gallons/minute. This information can be displayed to an operator or transmitted to a Programmable Logic Controller (PLC) used to control other equipment depending on the type of sensor and control system.
Figure 2. A flow meter used to continuously monitor actual flow rate in the water distribution system and relay information back to a central monitoring station.
Using UV in an example again, if a UV unit is designed to operate within a flow range of 60 to 80 gallons/minute to provide a correct treatment dosage, the flow meter allows the operator to monitor the actual flow entering the UV unit. If the water flow falls outside the desired operating range, the flow meter can provide a signal to a PLC, allowing for corrective action either manually or automatically.
In some applications, both devices may be used together. A flow meter can provide continuous flow information while a flow switch provides an independent safety factor. Ultimately, the appropriate device depends on whether the application requires a simple flow/no-flow signal or continuous measurement of the actual flow rate. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com to determine if a flow switch and/or flow meter may be appropriate technology for your equipment and aquatic system.